Method for making a lithographic printing plate

ABSTRACT

A method for making a lithographic printing plate includes the steps of: (1) providing a heat-sensitive lithographic printing plate precursor including on a support having a hydrophilic surface or which is provided with a hydrophilic layer, a heat-sensitive coating, (2) image-wise exposing the precursor with IR-radiation or heat, and (3) developing the image-wise exposed precursor with an alkaline developing solution including a compound having at least two onium groups. According to the above method, a printing plate is formed with an improved developing latitude or an improved exposure latitude.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application of PCT/EP2007/052443, filed Mar. 15, 2007. This application claims the benefit of U.S. Provisional Application No. 60/784,414, filed Mar. 21, 2006, which is incorporated by reference herein in its entirety. In addition, this application claims the benefit of European Application No. 06111310.6, filed Mar. 17, 2006, which is also incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for making a lithographic printing plate wherein an image-wise exposed precursor is developed with an alkaline developing solution including a compound having at least two onium groups. The present invention relates also to an alkaline developing solution and to a replenishing solution including the compound having at least two onium groups. The present invention relates also to a lithographic printing plate precursor wherein the coating includes the compound having at least two onium groups.

2. Description of the Related Art

Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.

Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. A typical positive-working plate precursor includes a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), heat-sensitive printing plate precursors have also become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.

Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support.

Typically, for a positive-working thermal plate, a dissolution inhibitor is added to a phenolic resin as a binder whereby the rate of dissolution of the coating is reduced. Upon heating, this reduced rate of dissolution of the coating is increased on the exposed areas compared with the non-exposed areas, resulting in a sufficient difference in solubility of the coating after image-wise recording by heat or IR-radiation. Many different dissolution inhibitors are known and disclosed in the literature, such as organic compounds having an aromatic group and a hydrogen bonding site or polymers or surfactants including siloxane or fluoroalkyl units.

The known heat-sensitive printing plate precursors typically include a hydrophilic support and a coating which is alkali-soluble in exposed areas (positive working material) or in non-exposed areas (negative working material) and an IR-absorbing compound. Such a coating typically includes an oleophilic polymer which may be a phenolic resin such as novolac, resol or a polyvinylphenolic resin. The phenolic resin can be chemically modified whereby the phenolic monomeric unit is substituted by a group such as described in WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445, and EP 2 102 446. The phenolic resin can also been mixed with other polymers such as an acidic polyvinyl acetal as described in WO 2004/020484 or a copolymer including sulfonamide groups as described in U.S. Pat. No. 6,143,464. The use of other polymeric binders in lithographic printing plates are described in WO 2001/09682, EP 933 682, WO 99/63407, WO 2002/53626, EP 1 433 594 and EP 1 439 058.

The positive-working thermal plate may further include, between the heat-sensitive recording layer and the support, an intermediate layer including an alkali soluble resin. This intermediate layer can improve the chemical resistance of the plate against press chemicals. This layer induces an improved removal of the coating on the exposed areas. Typical examples of positive-working thermal plate materials having such a two layer structure are described in, e.g., EP 864420, EP 909657, EP-A 1011970, EP-A 1263590, EP-A 1268660, EP-A 1072432, EP-A 1120246, EP-A 1303399, EP-A 1311394, EP-A 1211065, EP-A 1368413, EP-A 1241003, EP-A 1299238, EP-A 1262318, EP-A 1275498, EP-A 1291172, WO 2003/74287, WO 2004/33206, EP-A 1433594 and EP-A 1439058. However, in the non-exposed areas of these plates, the resistance of the coating for the alkaline developer is poor whereby the difference in the dissolution rate between the exposed and non-exposed areas is insufficient, i.e., the non-exposed areas are partially affected by the developer before the exposed areas are completely dissolved in the developer. As a result, it is difficult to form highly sharp and clear images, particularly highlights, i.e., fine images including a dot pattern or fine lines, are difficult to be reproduced.

In a high quality plate, it is advantageous that such highlights can be reproduced within a sufficient developing latitude, i.e., small fluctuations in developing time do not substantially affect the image formed on the plates and this developing latitude is obtained when the difference in the dissolution rate is improved.

EP 1 182 512 discloses an alkaline developing solution including an amphoteric or cationic surfactant for development of an infrared radiation-presensitized plate.

EP 1 400 856 discloses a method for making a lithographic printing plate whereby the exposed precursor is developed with an alkaline developing solution including a cationic surfactant or a compound having three or more ethylene oxide-terminal groups in the molecule thereof.

EP 1 211 065 discloses a positive-working heat-sensitive printing plate precursor wherein the recording layer includes an organic quaternary ammonium salt.

The printing plates of the prior art suffer from an insufficient developing latitude.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a method for making a heat-sensitive lithographic printing plate whereby excellent printing properties are obtained and whereby the developing latitude or exposure latitude are improved. A preferred embodiment of the present invention has the characteristic feature that the developing solution in the developing step includes a compound having at least two onium groups. This compound, hereinafter also referred to as a “blocking agent”, which is capable of reducing the solubility of the alkaline soluble resin of the coating of a positive-working heat-sensitive printing plate precursor whereby the resistance of the coating in the non-exposed areas against the alkaline developer is increased, is present in the precursor and/or in an alkaline developing solution or replenishing solution. By use of compounds mentioned in the prior art documents EP 1 182 512, EP 1 400 856 and EP 1 211 065 an insufficient developing latitude is obtained. Therefore, the inventors have discovered this new compound which is capable of improving the resistance of the coating in the non-exposed areas against the alkaline developer and is capable of improving the developing latitude or the exposure latitude.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of the present invention, there is provided a method for making a lithographic printing plate including the steps of: (1) providing a heat-sensitive lithographic printing plate precursor including on a support having a hydrophilic surface or which is provided with a hydrophilic layer, a heat-sensitive coating, (2) image-wise exposing the precursor with IR-radiation or heat, and (3) developing the image-wise exposed precursor with an alkaline developing solution including a compound having at least two onium groups.

In a preferred embodiment of the present invention, the compound having at least two onium groups has a structure according to Formula I:

wherein

-   Q is a divalent group Q² when e and f are 0, a trivalent group Q³     when the sum of e and f is 1, or a tetravalent group Q⁴ when e and f     are both 1; -   Z^(a) to Z^(d) independently are onium groups; -   L^(a) to L^(d) independently are divalent linking groups; and -   a, b, c, d, e and f independently are 0 or 1, preferably a, b, c and     d are 1.

In another preferred embodiment of the present invention, the compound having at least two onium groups has a structure according to Formula II:

wherein

-   Z^(a) and Z^(b) independently are onium groups; -   L^(a) is a divalent linking group, -Z^(x)-L^(x)- represents a     repeating unit wherein Z^(x) and L^(x) represent respectively a     divalent onium group and a divalent linking group for each x-value;     and -   x is 0, 1 or an integer >1, more preferably x is an integer ranging     between 1 and 50, most preferably between 1 and 10.

In a more preferred embodiment of the present invention, the compound having at least two onium groups has a structure according to Formula III: Z^(a)-L-Z^(b),  Formula III wherein

-   L is a divalent linking group; and -   Z^(a) and Z^(b) independently are onium groups.

The divalent linking groups L^(a) to L^(d), each L^(x) and L in the compounds of Formula I, II and III are preferably selected from the group consisting of a linear, branched or cyclic alkylene group such as a methylene, ethylene, propylene, butylene group, c-hexylene or 1-methyl-ethylene; an arylene group such as a phenylene, naphtalene or an heteroarylene group; an arylene-alkylene group such as benzylidene group; a bis-alkylene aromatic hydrocarbon group such as o-, p- or m-bis-methylene benzene group; an oxy-alkylene group such as methylene oxide, ethylene oxide or propylene oxide group; an oxy-arylene or oxy-heteroarylene group such as phenylene oxide; or a combination of two or more of these groups. Each of these alkylene or arylene groups can be substituted. The linking group of a preferred embodiment of the present invention is composed of at least one of these groups. In a preferred embodiment, the linking group is composed of one or two of these groups which are repeated for at least two times, more preferably for a number ranging between 2 and 15, most preferably between 3 and 8. The linking group of a preferred embodiment of the present invention can also be composed of two, three or more sequences of these groups wherein each sequence contains at least two or more of these groups.

Examples of a divalent linking group are —(CH₂)—, —(CH₂)—(CH₂)—, —(CH₂)—(CH—CH₃)—, —(CR^(a)R^(b))_(n)—, —C—C₆H₁₀—, —(C₆H₄)—, —(CH₂)—(C₆H₄)—, —(CH₂)—(C₆H₄)—(CH₂)—, —O—(CH₂)—, —O—(CH₂)—(CH₂)—, —O—(CH₂)—(CH—CH₃)—, —(C₆H₄)—(C₆H₄)—, —O—(C₆H₄)—, —(CH₂)—(CH₂)—[—O—(CH₂)—(CH₂)—]_(m)—,

wherein R^(a) and R^(b) are independently hydrogen or an optionally substituted alkyl or aryl group; n is an integer ranging between 2 and 15, preferably between 3 and 10, m is an integer ranging between 2 and 15, preferably between 3 and 10.

Q² is a divalent group, preferably selected from the group consisting of the divalent linking groups as defined above.

Q³ is a trivalent group, preferably selected from the group consisting of —CR′<, CR′(-(L′)_(p)—)(-(L″)_(q)-)(-(L′″)_(r)-), —(C₆H₃)<, C₆H₃(-(L′)_(p)-)(-(L″)_(q)-)(-L′″)_(r)-), [—S<]⁺, [—NR′<]⁺, or [—PR′<]⁺;

wherein

-   each of > and < represents two binding sites; -   R′, R″ and R′″ are hydrogen or an optionally substituted alkyl, aryl     or aralkyl group; -   L′, L″ and L′″ are a divalent linking group, preferably selected     from the group consisting of the divalent linking groups as defined     above; and -   p, q and r independently are 0 or 1.

Q⁴ is a tetravalent group, preferably selected from the list of the following groups: >C<, C(-(L′)_(p)—)(-(L″)_(q)—)(-(L′″)_(r)-) (-(L″″)_(s)-), >(C₆H₂)<, C₆H₂(-(L′)_(p)-)(-(L″)_(q)-)(-(L′″)_(r)-)(-(L″″)_(s)-), [>N<]⁺, or [>P<]⁺;

wherein

-   each of > and < represents two binding sites; -   L′, L″ and L′″ are a divalent linking group, preferably selected     from the group consisting of the divalent linking groups as defined     above; and -   p, q, r and s independently are 0 or 1, preferably 1.

The onium groups Z^(a) to Z^(d) and Z^(x) in the compounds of Formula I, II and III are positively charged groups and preferably selected from the group consisting of primary amine salts, secondary amine salts, tertiary amine salts, quaternary ammonium salts, phosphonium salts or sulphonium salts.

The onium groups Z^(a) to Z^(d) are more preferably selected from the groups consisting of one of the following structures:

wherein

-   * denotes the binding site of the onium group with one site of the     divalent linking group; -   X represents oxygen, sulfur or —NR²—; -   each of R¹ to R⁵ groups are independently hydrogen; an optionally     substituted alkyl, alkylene, aryl or aralkyl group such as methyl,     ethyl, propyl, butyl, propenyl, butenyl, phenyl or benzyl; a halogen     atom such as bromide or chloride; —CN; —NO₂; an oxy-alkyl or     oxy-aryl group; a thio-alkyl or thio-aryl group; an amide or     sulphonamide group; a carboxylic acid or salt or alkyl ester group;     a sulphonic acid or salt or alkyl ester group; a sulphonalkyl or     sulphonaryl group; an amino group; or whereby two groups of the R¹     to R⁵ groups together include the necessary atoms to form a cyclic     structure, preferably a 5- or 6-membered ring; -   each of R⁶ to R⁸ groups are independently hydrogen; an optionally     substituted alkyl, alkylene, aryl or aralkyl group such as methyl,     ethyl, propyl, butyl, propenyl, butenyl, phenyl or benzyl; a     carboxylic acid or salt or alkyl ester group; or whereby two groups     of the R⁶ to R⁸ groups together include the necessary atoms to form     a cyclic structure, preferably a 5- or 6-membered ring.

Each of the onium groups Z^(x) in the compounds of Formula II are positively charged groups having two binding sites, i.e., divalent, and more preferably selected from the list of the onium groups as above wherein one of the R¹ to R⁸ groups is not present and is replaced by *, indicating the second binding site.

In order to obtain electrical neutrality, one or more anions are present as a counterion. Examples of such counterions are chloride, bromide, iodide, or a compound including at least one of the following anionic groups: a sulphonate, sulphate, carboxylate, phosphate or phosphonate anion, or wherein the sulphonate, sulphate, carboxylate, phosphate or phosphonate anion is part of one of the substituting groups R¹ to R⁵ of the onium group as defined above. Preferred counterions are bromide, chloride, methyl sulphonate or tosylate.

The onium groups Z^(a) to Z^(d) in the compounds of Formula I preferably have the same structure.

The onium groups Z^(a) and Z^(b) in the compounds of Formula II preferably have the same structure.

All the Z^(x) groups in the compounds of Formula II preferably have the same structure.

The onium groups Z^(a) and Z^(b) in the compounds of Formula III preferably have the same structure.

Examples of blocking agents of preferred embodiments of the present invention are:

During developing of an image-wise exposed heat-sensitive lithographic printing plate precursor, the blocking agent of a preferred embodiment of the present invention is present in the aqueous alkaline developing solution.

In accordance with a preferred embodiment of the present invention, the blocking agent is present in the alkaline developing solution at the start of the developing process.

In accordance with another preferred embodiment of the present invention, the blocking agent is added to the alkaline developing solution during the developing process, preferably as a solution or dispersion. In a more preferred embodiment, the blocking agent is added to the alkaline developing solution by the addition of a replenishing solution including the blocking agent during the developing process.

In accordance with another preferred embodiment of the present invention, the blocking agent is added to the alkaline developing solution during the developing process by removing at least a portion of the precursor including the blocking agent. The blocking agent can be present in the heat-sensitive coating, an intermediate layer between the support and the heat-sensitive coating, a top layer and/or a layer on the back side of the plate.

Developing Solution

The aqueous alkaline developing solution (hereinafter also referred to as “developer”) may include an alkaline agent. The composition of the aqueous alkaline solution can be selected from conventional alkaline developers.

The aqueous alkaline developing solution of a preferred embodiment of the present invention preferably has a pH value of at least 10, more preferably of at least 11.5, most preferably of at least 12. There is no specific upper limit for the pH but the pH is usually not higher than 14.

The alkaline agent preferably includes inorganic alkaline agents such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium tertiary phosphate, potassium tertiary phosphate, ammonium tertiary phosphate, sodium secondary phosphate, potassium secondary phosphate, ammonium secondary phosphate, sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, sodium borate, potassium borate and ammonium borate, and potassium citrate, sodium citrate and the like.

The alkaline agent may also include organic alkaline agents such as monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monoisopropylamine, diisopropylamine, triisopropylamine, n-butylamine, monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, ethylenediamine, pyridine and the like.

The alkaline aqueous solution may also include an alkaline silicate. The alkaline silicate may be those exhibiting an alkalinity when dissolved in water, and examples thereof include an alkali metal silicate and alkali metal metasilicate such as sodium silicate, sodium metasilicate, potassium silicate and lithium silicate, and ammonium silicate. The alkaline silicate may be used alone, or in combination.

The development performance of the alkaline aqueous solution may be easily modulated by adjusting a molar ratio of alkaline silicates and alkali metal hydroxides, represented by silicon oxide (SiO₂) and an alkali oxide (M₂O, wherein M represents an alkali metal or an ammonium group). The alkaline aqueous solution preferably has a molar ratio SiO₂/M₂O from 0.5 to 3.0, and more preferably from 1.0 to 2.0. When the molar ratio SiO₂/M₂O is less than 0.5, alkalinity of the solution strengthens so as to cause a harmful effect such as etching of an aluminum plate which is generally used as a substrate in a lithographic printing plate precursor. When the molar ratio, SiO₂/M₂O is more than 3.0, the development performance of the solution may be degraded.

The concentration of alkaline silicate in the developer ranges generally from 1 to 14% by weight, preferably from 3 to 14% by weight, and more preferably from 4 to 14% by weight. When the concentration is less than 1% by weight, the development performance or treatment capacity may be degraded. When the concentration is more than 14% by weight, precipitated materials or crystals may be easily generated, and gelation may be easily caused during neutralization of waste liquid, resulting in an obstacle to the waste disposal.

The developer based on an alkaline aqueous solution may also include a nonreducing sugar. The nonreducing sugar denotes sugars having no reductive property due to the absence of a free aldehyde group or a free ketone group. The nonreducing sugar is classified into trehalose-type oligosaccharides wherein a reductive group and another reductive group make a linkage; glycosides wherein a reductive group in a sugar is linked to a non-sugar compound; and sugar alcohols which are produced by reducing a sugar by hydrogenation. The trehalose-type oligosaccharides include sucrose and trehalose, and the glycosides include alkyl glycosides, phenol glycosides, mustard oil glycosides and the like. The sugar alcohols include D,L-arabitol, ribitol, xylitol, D,L-sorbitol, D,L-mannitol, D,L-iditol, talitol, dulcitol, allodulcitol and the like. Further, maltitol obtained by hydrogenation of disaccharide, a reduced material obtained by hydrogenation of oligosaccharide (a reduced starch syrup) and the like are preferably used. In the above nonreducing sugar, preferred are sugar alcohols and sucrose, and particularly preferred are D-sorbitol, sucrose and a reduced starch syrup, since they have buffering action in appropriate pH range.

The above nonreducing sugar may be used alone or in combination with other ingredients, and the concentration thereof in the developer ranges generally from 0.1 to 30% by weight, and preferably from 1 to 20% by weight.

In the developer, an alkaline agent may be used as a base in combination with the above mentioned alkaline silicate or nonreducing sugar, and the alkaline agent may be selected from sodium hydroxide and potassium hydroxide. Further, sodium tertiary phosphate, potassium tertiary phosphate, sodium carbonate and potassium carbonate are also preferable, since they have themselves a buffering action. The above alkaline agent may be used alone or in combination.

The developer may optionally contain further components, such as buffer substances, complexing agents, antifoaming agents, organic solvents, antisludge agent, corrosion inhibitors, dyes, surfactants and/or hydrotropic agents as known in the art.

In the aqueous alkaline developing solution, it is possible to simultaneously use organic solvents having solubility in water at 20° C. of not more than 15% by weight according to need. Examples of such organic solvents are such carboxilic acid esters as ethyl acetate, propyl acetate, butyl acetate, amyl acetate, benzyl acetate, ethylene glycol monobutyl ether acetate, butyl lactate and butyl levulinate; such ketones as ethyl butyl ketone, methyl isobutyl ketone and cyclohexanone; such alcohols as ethylene glycol monobutyl ether, ethylene glycol benzyl ether, ethylene glycol monophenyl ether, benzyl alcohol, methylphenylcarbitol, n-amyl alcohol and methylamyl alcohol; such alkyl-substituted aromatic hydrocarbons as xylene; and such halogenated hydrocarbons as methylene dichloride and monochlorobenzene. These organic solvents may be used alone or in combination. Particularly preferred is benzyl alcohol. These organic solvents are added to the developer or replenisher therefor generally in an amount of not more than 10% by weight and preferably not more than 5% by weight.

Besides the components described above, the developing solution of a preferred embodiment of the present invention includes a non-ionic surfactant. Such a non-ionic surfactant may be used as a dissolution preventing agent whereby undercutting of non-exposed areas is reduced. Examples of nonionic surfactants include ethoxylated alcohols, polyethylene glycols and polyhydric alcohols. Ethoxylated alcohols are preferred, and in particular, ethoxylated alcohols having a hydrophilic-lipophilic balance (HLB) higher than 5 are more preferred. Specific examples of nonionic surfactants are listed below: polyethylene glycol, polyoxyethylene lauryl ether, laurylalcohol polyglycol ethers, oleyl alcohol ethoxylated, lanolin alcohol ethoxylated, dodecanol ethoxylated, cetostearyl alcohol ethoxylated, tetramethyldecindiol ethoxylated, polyoxyethylene nonyl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene behenyl ether, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene cetyl ether, polyoxyethylene polyoxypropylene behenyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene stearylamine, polyoxyethylene oleylamine, polyoxyethylene stearic acid amide, polyoxyethylene oleic acid amide, polyoxyethylene castor oil, polyoxyethylene abietyl ether, polyoxyethylene lanolin ether, polyoxyethylene monolaurate, polyoxyethylene monostearate, polyoxyethylene glyceryl monooleate, polyoxyethylene glyceryl monostearate, polyoxyethylene propylene glycol monostearate, polyoxyethylene sorbitan monolaurate, and sorbitan monolaurate. The non-ionic surfactants, which have an ethylene diamine group, substituted with groups of polyoxyethylene, polyoxypropylene or mixtures thereof, are of special interest in the developing solution. Examples of such non-ionic surfactants are the TETRONIC surfactants commercially available from BASF.

The non-ionic surfactants preferably have an average molecular weight ranging from 500 to 10,000, preferably from 1,000 to 5,000. They can be included individually or as admixture of two or more compounds in an amount ranging from 0.05 to 5%, preferably from 0.1 to 1% by weight of the total developing solution.

The developers used in a preferred embodiment of the present invention may simultaneously contain another surfactant for the purpose of improving developing properties thereof. By incorporating surfactants to the developer solution, the surface tension reduces drastically and the developer solution becomes effective, i.e., removal of the non-image areas without occurrence of stain/toning, at a lower pH compared to the same developer solution without a surfactant. The developer may include an ionic surfactant such as an anionic or cationic surfactant, a non-ionic surfactant and/or amphoteric surfactants such as Librateric AA30 (trademark from Libra Chemicals Limited). Examples of such surfactants include salts of higher alcohol (C8˜C22) sulfuric acid esters such as sodium salt of lauryl alcohol sulfate, sodium salt of octyl alcohol sulfate, ammonium salt of lauryl alcohol sulfate, Teepol B-81 (trade mark, available from Shell Chemicals Co., Ltd.) and sodium alkyl sulfates; salts of aliphatic alcohol phosphoric acid esters such as sodium salt of cetyl alcohol phosphate; alkyl aryl sulfonic acid salts such as sodium salt of dodecylbenzene sulfonate, sodium salt of isopropylnaphthalene sulfonate, sodium salt of dinaphthalene disulfonate and sodium salt of metanitrobenzene sulfonate; sulfonic acid salts of alkylamides such as C₁₇H₃₃CON(CH₃)CH₂CH₂SO₃Na and sulfonic acid salts of dibasic aliphatic acid esters such as sodium dioctyl sulfosuccinate and sodium dihexyl sulfosuccinate. These surfactants may be used alone or in combination. Particularly preferred are sulfonic acid salts. These surfactants may be used in an amount of generally not more than 5% by weight and preferably not more than 3% by weight.

The cationic surfactant used in the developer may include amine salts, quaternary ammonium salts, phosphonium salts, sulfonium salts and the like. Examples of the amine salts are primary amine salts, secondary amine salts, tertiary amine salts, and imidazoline salts. Examples of the quaternary ammonium salts include tetraalkyl quaternary ammonium salts, trialkyl benzyl quaternary ammonium salts, alkyl pyridinium salts, alkyl quinolinium salts, imidazolinium salts and benzimidazolinium salts.

Exemplary amphoteric surfactants include betaine derivatives, such as, for example, alkylamidopropyl betaine, alkyldimethyl betaine, bishydroxyethyl betaine, alkylamido propyl betaine, lauryl betaine, and the like, glycine derivatives, such as, for example, cocoamphocarboxy glycinate, lauroamphocarboxy glycinate, caprylamphocarboxy glycinate, oleoamphocarboxy glycinate, oleoamphopolycarboxy glycinate, N-alkyl glycinate, and the like, imino derivatives, such as, for example, cocoiminodipropionate, octyliminodipropionate, and the like, imidazoline derivatives, such as, for example, coconut imidazoline, and the like, lecithin derivatives, and aminocarboxylic acids. These amphoteric surfactants preferably have an average molecular weight ranging from 100 to 10,000, preferably from 1,000 to 5,000. They can be included individually or as an admixture of two or more compounds.

The above described anionic and amphoteric surfactant can be present in the developing solution in a total amount ranging from 0.5 to 10%, preferably from 1 to 6%, and more preferably from 1 to 3% by weight of the total developing solution.

In order to enhance developing stability of the developers, the following compounds may simultaneously be used. Examples of such compounds are neutral salts such as NaCl, KCl and KBr as disclosed in JP-A-58-75152; chelating agents such as EDTA and NTA as disclosed in JP-A-58-190 952 (U.S. Pat. No. 4,469,776), complexes such as [Co(NH₃)₆]Cl₃ as disclosed in JP-A-59-121336 (U.S. Pat. No. 4,606,995); ionizable compounds of elements of the group IIa, IIIa or IIIb of the Periodic Table such as those disclosed in JP-A-55-25100; anionic or amphoteric surfactants such as sodium alkyl naphthalene sulfonate and N-tetradecyl-N,N-dihydroxyethyl betaine as disclosed in JP-A-50-51324; tetramethyldecyne diol as disclosed in U.S. Pat. No. 4,374,920; non-ionic surfactants as disclosed in JP-A-60-213943; cationic polymers such as methyl chloride quaternary products of p-dimethylaminomethyl polystyrene as disclosed in JP-A-55-95946; amphoteric polyelectrolytes such as copolymer of vinylbenzyl trimethylammonium chloride and sodium acrylate as disclosed in JP-A-56-142528; reducing inorganic salts such as sodium sulfite as disclosed in JP-A-57-192952 (U.S. Pat. No. 4,467,027) and alkaline-soluble mercapto compounds or thioether compounds such as thiosalicylic acid, cysteine and thioglycolic acid; inorganic lithium compounds such as lithium chloride as disclosed in JP-A-58-59444; organic lithium compounds such as lithium benzoate as disclosed in JP-A-50-34442; organometallic surfactants containing Si, Ti or the like as disclosed in JP-A-59-75255; organoboron compounds as disclosed in JP-A-59-84241 (U.S. Pat. No. 4,500,625); quaternary ammonium salts such as tetraalkylammonium oxides as disclosed in EP-A-101 010; and bactericides such as sodium dehydroacetate as disclosed in JP-A-63-226657.

In a preferred embodiment of the present invention, the blocking agent is present in a developing solution at a concentration of at least 0.01 g/l, preferably in a concentration ranging between 0.05 and 30 g/l, more preferably between 0.05 and 15 g/l, more preferably between 0.05 and 5 g/l, most preferably between 0.1 and 1 g/l.

Replenishing Solution

During the development, the non-image areas of the coating are removed with an aqueous alkaline developer solution optionally in combination combined with mechanical rubbing, e.g., by a rotating brush. During the development step, any water-soluble layer present is preferably also removed. The development is preferably carried out at temperatures of from 20 to 40° C. in automated processing units as customary in the art.

To perform development processing stably for a long time period, it is particularly important to control the strength of alkali and the concentration of silicates and other ingredients in the developer. Therefore, a replenishing solution, hereinafter also referred to as replenisher, is added to the developing solution, usually in small amounts such that a stable development processing can be performed over a long time period without exchanging the developer. Therefore, it is also important that the concentration of the blocking agent is present in the developing solution at a sufficiently high level in order to maintain the developing latitude or exposure latitude of the precursor and this can be realized by adding a replenishing solution including the blocking agent of a preferred embodiment of the present invention.

For regeneration, the replenishing solution, hereinafter also referred to as replenisher, preferably includes active ingredients of the developing solution such as alkaline agent. The replenishing solution is added continuously or in small amounts to the developing solution during or after the developing process in order to regulate the concentration of the active ingredients in the developing solution at a level sufficiently high or at a constant level such that the development of image-wise exposed precursors remains at a constant level. The required amounts of regenerated material must be tailored to the developing apparatuses used, daily plate throughputs, image areas, etc., and are in general from 1 to 150 ml per square meter of plate precursor. The addition of replenisher can be regulated, for example, by measuring the conductivity of the developer as described in EP-A 0 556 690.

In the method for development processing, any known process of supplementing a replenisher for developer may also be employed. Examples of such methods preferably used are a method for intermittently or continuously supplementing a replenisher as a function of the amount of plates processed and time as disclosed in JP-A-55-115039 (GB-A-2 046 931), a method including disposing a sensor for detecting the degree of light-sensitive layer dissolved out in the middle portion of a developing zone and supplementing the replenisher in proportion to the detected degree of the light-sensitive layer dissolved out as disclosed in JP-A-58-95349 (U.S. Pat. No. 4,537,496); a method including determining the impedance value of a developer and processing the detected impedance value by a computer to perform supplementation of a replenisher as disclosed in GB-A-2 208 249.

In a preferred embodiment of the present invention, the blocking agent is present in a replenishing solution at a concentration of at least 0.05 g/l, preferably in a concentration ranging between 0.1 and 30 g/l, more preferably between 0.5 and 20 g/l, more preferably between 0.5 and 15 g/l, most preferably between 0.5 and 10 g/l. In another preferred embodiment, the ratio of the concentration of the blocking agent in the replenishing solution, containing at least 0.05 g/l of the blocking agent, and in the developing solution is preferably between 0.5 and 100, more preferably between 1.1 and 100, most preferably between 2 and 50.

The replenishing solution preferably has a pH value of at least 10, more preferably of at least 11.5, most preferably of at least 12. There is no specific upper limit for the pH but the pH is usually not higher than 14.

In another preferred embodiment of the present invention, more than one replenishing solution can be used for adding to the developing solution during or after the development processing and these replenishers may contain different types in different amounts of blocking agents, and also different types in different amounts of other compounds for improving the development processing.

In a more preferred embodiment of the present invention, the fresh developing solution at starting the development processing for the first time is essentially free of the blocking agent according to a preferred embodiment of the present invention and during or after processing precursors, a replenishing solution, containing the blocking agent, is added to the developing solution and/or a layer of the precursor, including the blocking agent, is removed at least partially in the developing solution.

The heat-sensitive lithographic printing plate precursor of a preferred embodiment of the present invention includes a support having a hydrophilic surface or which is provided with a hydrophilic layer, and, on the support, a heat-sensitive coating.

Support

The support of the lithographic printing plate precursor has a hydrophilic surface or is provided with a hydrophilic layer. The support may be a sheet-like material such as a plate or it may be a cylindrical element such as a sleeve which can be slid around a print cylinder of a printing press. A preferred support is a metal support such as aluminum or stainless steel. The metal can also be laminated to a plastic layer, e.g., polyester film.

A particularly preferred lithographic support is an electrochemically grained and anodized aluminum support. Graining and anodization of aluminum is well known in the art. The anodized aluminum support may be treated to improve the hydrophilic properties of its surface. For example, the aluminum support may be silicated by treating its surface with a sodium silicate solution at elevated temperature, e.g., 95° C. Alternatively, a phosphate treatment may be applied which involves treating the aluminum oxide surface with a phosphate solution that may further contain an inorganic fluoride. Further, the aluminum oxide surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or may be carried out at a slightly elevated temperature of about 30 to 50° C. A further interesting treatment involves rinsing the aluminum oxide surface with a bicarbonate solution. Still further, the aluminum oxide surface may be treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulfonated aliphatic aldehyde. It is further evident that one or more of these post treatments may be carried out alone or in combination. More detailed descriptions of these treatments are given in GB-A 1 084 070, DE-A 4 423 140, DE-A 4 417 907, EP-A 659 909, EP-A 537 633, DE-A 4 001 466, EP-A 292 801, EP-A 291 760 and U.S. Pat. No. 4,458,005.

Coating

The heat-sensitive coating, which is provided on the support, may be positive-working or negative-working. A positive-working heat-sensitive coating is preferred. The coating of a positive-working heat-sensitive coating does not dissolve in an alkaline developing solution in the unexposed areas and becomes soluble in the exposed areas within the time used for developing the plate. The coating preferably includes an infrared absorbing agent and an alkaline soluble oleophilic resin wherein the solubility in an alkaline developing solution is reduced in the coating and wherein the solubility in an alkaline developing solution is increased upon heating or IR-radiation. The coating preferably further includes a dissolution inhibitor whereby the rate of dissolution in an alkaline developing solution is reduced. Due to this solubility differential the rate of dissolution of the exposed areas is sufficiently higher than in the non-exposed areas.

In a preferred embodiment of the present invention, the heat-sensitive coating includes a first polymer which is a phenolic resin such as novolac, resol or a polyvinylphenolic resin; novolac is more preferred. Typical examples of such polymers are described in DE-A-4007428, DE-A-4027301 and DE-A-4445820. Other preferred polymers are phenolic resins wherein the phenyl group or the hydroxy group of the phenolic monomeric unit are chemically modified with an organic substituent as described in EP 894 622, EP 901 902, EP 933 682, WO 99/63407, EP 934 822, EP 1 072 432, U.S. Pat. No. 5,641,608, EP 982 123, WO 99/01795, WO 04/035310, WO 04/035686, WO 04/035645, WO 04/035687 or EP 1 506 858.

The novolac resin or resol resin may be prepared by polycondensation of at least one member selected from aromatic hydrocarbons such as phenol, o-cresol, p-cresol, m-cresol, 2,5-xylenol, 3,5-xylenol, resorcinol, pyrogallol, bisphenol, bisphenol A, trisphenol, o-ethylphenol, p-etylphenol, propylphenol, n-butylphenol, t-butylphenol, octyl-phenol, 1-naphtol and 2-naphtol, with at least one aldehyde or ketone selected from aldehydes such as formaldehyde, glyoxal, acetoaldehyde, propionaldehyde, benzaldehyde and furfural and ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, in the presence of an acid catalyst. Instead of formaldehyde and acetaldehyde, paraformaldehyde and paraldehyde may, respectively, be used.

The weight average molecular weight, measured by gel permeation chromatography using universal calibration and polystyrene standards, of the novolac resin is preferably from 500 to 150,000 g/mol, more preferably from 1,500 to 50,000 g/mol.

The poly(vinylphenol) resin may also be a polymer of one or more hydroxy-phenyl containing monomers such as hydroxystyrenes or hydroxy-phenyl(meth)acrylates. Examples of such hydroxystyrenes are o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, 2-(o-hydroxyphenyl)propylene, 2-(m-hydroxyphenyl)propylene and 2-(p-hydroxyphenyl)propylene. Such a hydroxystyrene may have a substituent such as chlorine, bromine, iodine, fluorine or a C₁₋₄ alkyl group, on its aromatic ring. An example of such hydroxy-phenyl(meth)acrylate is 2-hydroxy-phenyl methacrylate.

The poly(vinylphenol) resin may usually be prepared by polymerizing one or more hydroxy-phenyl containing monomer in the presence of a radical initiator or a cationic polymerization initiator. The poly(vinylphenol) resin may also be prepared by copolymerizing one or more of these hydroxy-phenyl containing monomers with other monomeric compounds such as acrylate monomers, methacrylate monomers, acrylamide monomers, methacrylamide monomers, vinyl monomers, aromatic vinyl monomers or diene monomers.

The weight average molecular weight, measured by gel permeation chromatography using universal calibration and polystyrene standards, of the poly(vinylphenol) resin is preferably from 1,000 to 200,000 g/mol, more preferably from 1,500 to 50,000 g/mol.

Examples of phenolic resins are:

-   POL-01: ALNOVOL SPN452 is a solution of a novolac resin, 40% by     weight in Dowanol PM, obtained from CLARIANT GmbH. -   Dowanol PM consists of 1-methoxy-2-propanol (>99.5%) and     2-methoxy-1-propanol (<0.5%). -   POL-02: ALNOVOL SPN400 is a solution of a novolac resin, 44% by     weight in Dowanol PMA, obtained from CLARIANT GmbH.     Dowanol PMA consists of 2-methoxy-1-methyl-ethylacetate. -   POL-03: ALNOVOL HPN100 a novolac resin obtained from CLARIANT GmbH. -   POL-04: DURITE PD443 is a novolac resin obtained from BORDEN CHEM.     INC. -   POL-05: DURITE SD423A is a novolac resin obtained from BORDEN CHEM.     INC. -   POL-06: DURITE SD126A is a novolac resin obtained from BORDEN CHEM.     INC. -   POL-07: BAKELITE 6866LB02 is a novolac resin obtained from BAKELITE     AG. -   POL-08: BAKELITE 6866LB03 is a novolac resin obtained from BAKELITE     AG. -   POL-09: KR 400/8 is a novolac resin obtained from KOYO CHEMICALS     INC. -   POL-10: HRJ 1085 is a novolac resin obtained from SCHNECTADY     INTERNATIONAL INC. -   POL-11: HRJ 2606 is a phenol novolac resin obtained from SCHNECTADY     INTERNATIONAL INC. -   POL-12: LYNCUR CMM is a copolymer of 4-hydroxy-styrene and methyl     methacrylate obtained from SIBER HEGNER.

In another preferred embodiment of the present invention, the heat-sensitive coating further includes a second polymer which is insoluble in water and soluble in an alkaline solution.

In accordance with a more preferred embodiment of the present invention, the heat-sensitive coating includes a heat-sensitive layer and an intermediate layer. The intermediate layer is present between the heat-sensitive layer and the hydrophilic surface of the support. In a still more preferred embodiment, the heat-sensitive layer includes a first polymer and optionally an inhibitor, and the intermediate layer includes a second polymer.

The second polymer is preferably an organic polymer which has acidic groups with a pKa of less than 13 to ensure that the layer is soluble or at least swellable in aqueous alkaline developers. Advantageously, the binder is a polymer or polycondensate, for example, a polyester, a polyamide resin, an epoxy resin, an acetal resin, an acrylic resin, a methacrylic resin, a styrene based resin, a polyurethane resin or polyurea.

More preferably, the second polymer has one or more functional groups selected from the list of:

-   -   (i) a sulfonamide group such as —SO₂—NH—R^(g) wherein R^(g)         represents a hydrogen or an optionally substituted hydrocarbon         group such as an optionally substituted alkyl, aryl or         heteroaryl group;     -   (ii) an active imide group such as —SO₂—NH—CO—R^(h),         —SO₂—NH—SO₂—R^(h) or —CO—NH—SO₂—R^(h) wherein R^(h) represents a         hydrogen or an optionally substituted hydrocarbon group such as         an optionally substituted alkyl, aryl or heteroaryl group;     -   (iii) a carboxylate group;     -   (iv) a sulfonate group;     -   (v) phosphonate group, and     -   (vi) a phosphate group;         a sulfonamide group or an active imide group are more preferred;         most preferred are polymers selected from a copolymer including         a N-benzyl-maleimide monomeric unit or a monomeric unit         including a sulfonamide group as described in EP-A 933 682, EP 0         894 622 (page 3, line 16 to page 6, line 30), EP-A 0 982 123         (page 3, line 56 to page 51, line 5), EP-A 1 072 432 (page 4,         line 21 to page 10, line 29) and WO 99/63407 (page 4, line 13 to         page 9, line 37).

Other polymers having an acidic group are polycondensates and polymers having free phenolic hydroxyl groups, as obtained, for example, by reacting phenol, resorcinol, a cresol, a xylenol or a trimethylphenol with aldehydes, especially formaldehyde, or ketones. Condensates of sulfamoyl- or carbamoyl-substituted aromatics and aldehydes or ketones are also suitable. Polymers of bismethylol-substituted ureas, vinyl ethers, vinyl alcohols, vinyl acetals or vinylamides and polymers of phenylacrylates and copolymers of hydroxy-phenylmaleimides are likewise suitable. Furthermore, polymers having units of vinylaromatics, N-aryl(meth)acrylamides or aryl(meth)acrylates may be mentioned, it being possible for each of these units also to have one or more carboxyl groups, phenolic hydroxyl groups, sulfamoyl groups or carbamoyl groups. Specific examples include polymers having units of 2-hydroxyphenyl(meth)acrylate, of N-(4-hydroxyphenyl)(meth)acrylamide, of N-(4-sulfamoylphenyl)-(meth)acrylamide, of N-(4-hydroxy-3,5-dimethylbenzyl)-(meth)acrylamide, or 4-hydroxystyrene or of hydroxyphenylmaleimide. The polymers may additionally contain units of other monomers which have no acidic units. Such units include vinylaromatics, methyl(meth)acrylate, phenyl(meth)acrylate, benzyl(meth)acrylate, methacrylamide or acrylonitrile.

Dissolution Inhibitor

In a preferred embodiment of the present invention, the heat-sensitive coating or the heat-sensitive layer also contain one or more dissolution inhibitors. Dissolution inhibitors are compounds which reduce the dissolution rate of the hydrophobic polymer in the aqueous alkaline developer at the non-exposed areas of the coating and wherein this reduction of the dissolution rate is destroyed by the heat generated during the exposure so that the coating readily dissolves in the developer at exposed areas. The dissolution inhibitor exhibits a substantial latitude in the dissolution rate between the exposed and non-exposed areas. By preference, the dissolution inhibitor has a good dissolution rate latitude when the exposed coating areas have dissolved completely in the developer before the non-exposed areas are attacked by the developer to such an extent that the ink-accepting capability of the coating is affected. The dissolution inhibitor(s) can be added to the layer which includes the hydrophobic polymer discussed above.

The dissolution rate of the non-exposed coating in the developer is preferably reduced by interaction between the hydrophobic polymer and the inhibitor, due to, e.g., hydrogen bonding between these compounds. Suitable dissolution inhibitors are preferably organic compounds which include at least one aromatic group and a hydrogen bonding site, e.g., a carbonyl group, a sulfonyl group, or a nitrogen atom which may be quaternized and which may be part of a heterocyclic ring or which may be part of an amino substituent of the organic compound. Suitable dissolution inhibitors of this type have been disclosed in, e.g., EP-A 825 927 and 823 327.

Water-repellent polymers represent another type of suitable dissolution inhibitors. Such polymers seem to increase the developer resistance of the coating by repelling the aqueous developer from the coating. The water-repellent polymers can be added to the layer including the first polymer and/or can be present in a separate layer provided on top of the layer with the first polymer. In the latter preferred embodiment, the water-repellent polymer forms a barrier layer which shields the coating from the developer and the solubility of the barrier layer in the developer or the penetrability of the barrier layer by the developer can be increased by exposure to heat or infrared light, as described in, e.g., EP-A 864420, EP-A 950 517 and WO 99/21725. Preferred examples of the water-repellent polymers are polymers including siloxane and/or perfluoroalkyl units. In a preferred embodiment, the coating contains such a water-repellent polymer in an amount between 0.5 and 25 mg/m², preferably between 0.5 and 15 mg/m² and most preferably between 0.5 and 10 mg/m². When the water-repellent polymer is also ink-repelling, e.g., in the case of polysiloxanes, higher amounts than 25 mg/m² can result in poor ink-acceptance of the non-exposed areas. An amount lower than 0.5 mg/m² on the other hand may lead to an unsatisfactory development resistance. The polysiloxane may be a linear, cyclic or complex cross-linked polymer or copolymer. The term polysiloxane compound shall include any compound which contains more than one siloxane group —Si(R,R′)—O—, wherein R and R′ are optionally substituted alkyl or aryl groups. Preferred siloxanes are phenylalkylsiloxanes and dialkylsiloxanes. The number of siloxane groups in the (co)polymer is at least 2, preferably at least 10, more preferably at least 20. It may be less than 100, preferably less than 60. In another preferred embodiment, the water-repellent polymer is a block-copolymer or a graft-copolymer of a poly(alkylene oxide) block and a block of a polymer including siloxane and/or perfluoroalkyl units. A suitable copolymer includes about 15 to 25 siloxane units and 50 to 70 alkylene oxide groups. Preferred examples include copolymers including phenylmethylsiloxane and/or dimethylsiloxane as well as ethylene oxide and/or propylene oxide, such as Tego Glide 410, Tego Wet 265, Tego Protect 5001 or Silikophen P50/X, all commercially available from Tego Chemie, Essen, Germany. Such a copolymer acts as a surfactant which upon coating, due to its bifunctional structure, automatically positions itself at the interface between the coating and air and thereby forms a separate top layer even when the whole coating is applied from a single coating solution. Simultaneously, such surfactants act as a spreading agent which improves the coating quality. Alternatively, the water-repellent polymer can be applied in a second solution, coated on top of the layer including the hydrophobic polymer. In that preferred embodiment, it may be advantageous to use a solvent in the second coating solution that is not capable of dissolving the ingredients present in the first layer so that a highly concentrated water-repellent phase is obtained at the top of the coating.

Development Accelerator

Preferably, also one or more development accelerators are included in the heat-sensitive coating or in the heat-sensitive layer, i.e., compounds which act as dissolution promoters because they are capable of increasing the dissolution rate of the non-exposed coating in the developer. The simultaneous application of dissolution inhibitors and accelerators allows a precise fine tuning of the dissolution behavior of the coating. Suitable dissolution accelerators are cyclic acid anhydrides, phenols or organic acids. Examples of the cyclic acid anhydride include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, tetrachlorophthalic anhydride, maleic anhydride, chloromaleic anhydride, alpha-phenylmaleic anhydride, succinic anhydride, and pyromellitic anhydride, as described in U.S. Pat. No. 4,115,128. Examples of the phenols include bisphenol A, p-nitrophenol, p-ethoxyphenol, 2,4,4′-trihydroxybenzophenone, 2,3,4-trihydroxy-benzophenone, 4-hydroxybenzophenone, 4,4′,4″-trihydroxy-triphenylmethane, and 4,4′,3″,4″-tetrahydroxy-3,5,3′,5′-tetramethyltriphenyl-methane, and the like. Examples of the organic acids include sulfonic acids, sulfinic acids, alkylsulfuric acids, phosphonic acids, phosphates, and carboxylic acids, as described in, for example, JP-A 60-88942 and JP-A 2-96755. Specific examples of these organic acids include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, p-toluenesulfinic acid, ethylsulfuric acid, phenylphosphonic acid, phenylphosphinic acid, phenyl phosphate, diphenyl phosphate, benzoic acid, isophthalic acid, adipic acid, p-toluic acid, 3,4-dimethoxybenzoic acid, phthalic acid, terephthalic acid, 4-cyclohexene-1,2-dicarboxylic acid, erucic acid, lauric acid, n-undecanoic acid, and ascorbic acid. The amount of the cyclic acid anhydride, phenol, or organic acid contained in the coating is preferably in the range of 0.05 to 20% by weight, relative to the coating as a whole.

Blocking Agent

In accordance with a preferred embodiment of the present invention, the blocking agent is added to the developing solution by removing at least a portion of the precursor wherein the blocking agent of a preferred embodiment of the present invention is present. In this preferred embodiment, the layer including the blocking agent is removed at least partially during development, preferably, the blocking agent is present in the heat-sensitive coating or in the heat-sensitive layer and/or the intermediate layer and/or in a top layer and/or in an optional other layer on the back side of the support.

According to a more preferred embodiment, the blocking agent is present in the heat-sensitive coating or in the heat-sensitive layer and/or the intermediate layer whereby the layer including the blocking agent is removed at the exposed areas in the developing solution. In this way, it is highly advantageous that the amount of blocking agent added to the developing solution is in correspondence with the amount of precursors developed. When the precursor has a toplayer which is removed only on the exposed areas, the blocking agent can also be present in the toplayer and the addition of blocking agent is also in correspondence with the exposed areas. In this way, the blocking agent is released in proportion with exposed areas and the amount of blocking agent present in such an image-wise dissolving coating ranges preferably between 0.01 g/m² to 2.5 g/m², more preferably between 0.05 g/m² to 1.5 g/m², most preferably between 0.07 g/m² to 1.0 g/m².

According to another preferred embodiment of the present invention, the blocking agent is present in a layer on the back side of the support and this layer is removed at least partially in the developing solution. In this way, the addition of the blocking agent is proportional with the surface of the precursors which are developed. In an analogous way, the blocking agent can also be present in a top layer which is removed also in the non-exposed areas. In this way, the blocking agent is released in proportion with a surface of the precursor and the amount of blocking agent present in such a layer ranges preferably between 0.01 g/m² to 2.0 g/m², more preferably between 0.05 g/m² to 1.5 g/m², most preferably between 0.07 g/m² to 1.0 g/m².

In order to obtain an optimum blocking activity, it is advantageous to combine a precursor, having the blocking agent in a coating in a lower concentration, preferably between 0.005 and 0.35 g/m², with a developing solution (or replenisher), having a concentration of the blocking agent, preferably in the range of 0.1 and 3 g/l.

Exposure

The material can be image-wise exposed directly with heat, e.g., by a thermal head, or indirectly by infrared light, which is preferably converted into heat by an infrared light absorbing compound, which may be a dye or pigment having an absorption maximum in the infrared wavelength range. The infrared light absorbing dye or pigment is preferably present in the heat-sensitive coating or the heat-sensitive layer and typically in a concentration ranging between 0.25 and 10.0 wt. %, more preferably between 0.5 and 7.5 wt. % relative to the coating as a whole. Preferred IR-absorbing compounds are dyes such as cyanine or merocyanine dyes or pigments such as carbon black. A suitable compound is the following infrared dye IR-1:

wherein X⁻ is a suitable counter ion such as tosylate.

The heat-sensitive coating, or the heat-sensitive layer and/or the intermediate layer, may further contain an organic dye which absorbs visible light so that a perceptible image is obtained upon image-wise exposure and subsequent development. Such a dye is often called contrast dye or indicator dye. Preferably, the dye has a blue color and an absorption maximum in the wavelength range between 600 nm and 750 nm. Although the dye absorbs visible light, it preferably does not sensitize the printing plate precursor, i.e., the coating does not become more soluble in the developer upon exposure to visible light. Suitable examples of such a contrast dye are the quaternized triarylmethane dyes.

According to a preferred embodiment, the contrast dye is present in the heat-sensitive coating, or the heat-sensitive layer and/or the intermediate layer.

According to a highly preferred embodiment, the infrared light absorbing compound is concentrated in the heat-sensitive coating or the heat-sensitive layer.

The printing plate precursor of a preferred embodiment of the present invention can be exposed to infrared light with LEDs or a laser. Preferably, a laser emitting near infrared light having a wavelength in the range from about 750 to about 1500 nm is used, such as a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser. The required laser power depends on the sensitivity of the image-recording layer, the pixel dwell time of the laser beam, which is determined by the spot diameter (typical value of modern plate-setters at 1/e² of maximum intensity: 10-25 μm), the scan speed and the resolution of the exposure apparatus (i.e., the number of addressable pixels per unit of linear distance, often expressed in dots per inch or dpi; typical value: 1,000-4,000 dpi).

Two types of laser-exposure apparatuses are commonly used: internal (ITD) and external drum (XTD) plate-setters. ITD plate-setters for thermal plates are typically characterized by a very high scan speed up to 500 m/sec and may require a laser power of several Watts. XTD plate-setters for thermal plates having a typical laser power from about 200 mW to about 1 W operate at a lower scan speed, e.g., from 0.1 to 10 m/sec.

The known plate-setters can be used as an off-press exposure apparatus, which offers the benefit of reduced press down-time. XTD plate-setter configurations can also be used for on-press exposure, offering the benefit of immediate registration in a multi-color press. More technical details of on-press exposure apparatuses are described in e.g., U.S. Pat. Nos. 5,174,205 and 5,163,368.

In the development step, the non-image areas of the coating are removed by immersion in an aqueous alkaline developer, which may be combined with mechanical rubbing, e.g., by a rotating brush. The developer preferably has a pH above 10, more preferably above 12. The developer may further contain a poly hydroxyl compound such as, e.g., sorbitol, preferably in a concentration of at least 40 g/l, and also a polyethylene oxide containing compound such as, e.g., Supronic B25, commercially available from RODIA, preferably in a concentration of at most 0.15 g/l.

The development step may be followed by a rinsing step and/or a gumming step. The gumming step involves post-treatment of the lithographic printing plate with a gum solution. A gum solution is typically an aqueous liquid which includes one or more surface protective compounds that are capable of protecting the lithographic image of a printing plate against contamination or damaging. Suitable examples of such compounds are film-forming hydrophilic polymers or surfactants.

The plate precursor can, if required, be post-treated with a suitable correcting agent or preservative as known in the art. To increase the resistance of the finished printing plate and hence to extend the run length, the layer can be briefly heated to elevated temperatures (“baking”). The plate can be dried before baking or is dried during the baking process itself. During the baking step, the plate can be heated at a temperature which is higher than the glass transition temperature of the heat-sensitive coating, e.g., between 100° C. and 230° C. for a period of 40 seconds to 5 minutes. Baking can be done in conventional hot air ovens or by irradiation with lamps emitting in the infrared or ultraviolet spectrum. As a result of this baking step, the resistance of the printing plate to plate cleaners, correction agents and UV-curable printing inks increases. Such a thermal post-treatment is described, inter alia, in DE 1,447,963 and GB 1,154,749.

The printing plate thus obtained can be used for conventional, so-called wet offset printing, in which ink and an aqueous dampening liquid is supplied to the plate. Another suitable printing method uses so-called single-fluid ink without a dampening liquid. Suitable single-fluid inks have been described in U.S. Pat. Nos. 4,045,232; 4,981,517 and 6,140,392. In a preferred embodiment, the single-fluid ink includes an ink phase, also called the hydrophobic or oleophilic phase, and a polyol phase as described in WO 00/32705.

EXAMPLES Preparation of Binder-01

In a 250 ml reactor, 162 mmol of Monomer-01, 21.3 g (132 mmol) benzyl acrylamide, 0.43 g (6 mmol) acrylic acid and 103 g gamma-butyrolactone were added and the mixture was heated to 140° C., while stirring at 200 rpm. A constant flow of nitrogen was put over the reactor. After dissolution of all the components, the reactor was cooled to 100° C. 0.35 ml Trigonox DC50, commercially available from AKZO NOBEL, was added followed by the addition of 1.39 ml Trigonox 141, commercially available from AKZO NOBEL, in 3.43 ml butyrolactone. The polymerization was started and the reactor was heated to 140° C. over 2 hours while dosing 1.75 ml Trigonox DC50. The mixture was stirred at 400 rpm and the polymerization was allowed to continue for 2 hours at 140° C. The reaction mixture was cooled to 120° C. and the stirrer speed was enhanced to 500 rpm. 85.7 ml 1-methoxy-2-propanol was added and the reaction mixture was allowed to cool down to room temperature.

Binder-01 was analyzed with ¹H-NMR-spectroscopy and size exclusion chromatography, using dimethyl acetamide/0.21% LiCl as eluent on a 3× mixed-B column and relative to polystyrene standards.

M_(n) M_(w) PD Binder-01 23500 67000 2.84

The reaction mixture was cooled to 40° C. and the resulting 25 weight % polymer solution was collected in a drum.

Preparation of the Lithographic Support

A 0.30 mm thick aluminum foil was degreased by immersing the foil in an aqueous solution containing 34 g/l of sodium hydroxide at 70° C. for 6 seconds and rinsed with demineralized water for 3.6 seconds. The foil was then electrochemically grained for 8 seconds using an alternating current in an aqueous solution containing 15 g/l HCl, 15 g/l SO₄ ²⁻ ions and 5 g/l Al³⁺ at a temperature of 37° C. and a current density of 100 A/dm². The aluminum foil was then desmutted by etching with an aqueous solution containing 145 g/l of sulfuric acid at 80° C. for 5 seconds and rinsed with demineralized water for 4 seconds. The foil was subsequently subjected to anodic oxidation for 10 seconds in an aqueous solution containing 145 g/l of sulfuric acid at a temperature of 57° C. and a current density of 25 A/dm², then washed with demineralized water for 7 seconds and post-treated for 4 seconds with a solution containing 2.2 g/l of polyvinylphosphonic acid at 70° C., rinsed with demineralized water for 3.5 seconds and dried at 120° C. for 7 seconds.

The support thus obtained was characterized by a surface roughness Ra of 0.35-0.40 μm (measured with interferometyer NT1100) and an anodic weight of 3.0 g/m².

Preparation of the Printing Plate Precursor PPP-01

The PPP-01 was produced by first applying a first coating layer defined in Table 1 onto the above described lithographic support. The coating was applied at a wet coating thickness of 20 μm and then dried at 135° C.

TABLE 1 Composition of the Solution of the First Coating Layer Amount of product INGREDIENTS (g) Dowanol PM (1) 351.54 THF (2) 1698.99 Binder-01 (25% by weight) (3) 661.86 Crystal Violet (1% by weight) (4) 253.77 Tegoglide 410 (1% by weight) (5) 33.84 (1) Dowanol PM is 1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company. (2) THF is tetrahydrofuran. (3) Binder-01, preparation see above. (4) Crystal Violet, commercially available from CIBA-GEIGY. (5) TEGOGLIDE 410 is a copolymer of polysiloxane and poly(alkylene oxide), commercially available from TEGO CHEMIE SERVICE GmbH.

On the first coated layer, a second layer as defined in Table 2 was coated at a wet coating thickness of 25 μm and dried at 135° C.

TABLE 2 Composition of the Solution of the Second Coating Layer Amount of product INGREDIENTS (g) Dowanol PM (1) 600.73 Alnovol SPN452 (40% by weight) (2) 132.07 TMCA (10% by weight) (3) 68.20 SOO94 (4) 2.74 Crystal Violet (1% by weight) (5) 80.23 Tegowet 265 (1% by weight) (6) 11.23 Tegoglide 410 (1% by weight) (7) 36.90 Butanone 867.91 (1) See Table 1. (2) Alnovol SPN452 is a novolac solution, 40.5% by weight in Dowanol PM, commercially available from CLARIANT. (3) TMCA is 3,4,5-trimethoxy cinnamic acid. (4) SOO94 is an IR absorbing cyanine dye, commercially available from FEW CHEMICALS; the chemical structure of SOO94 is equal to IR-1 (5) See Table 1. (6) Tegowet 265 is a copolymer of polysiloxane and poly(alkylene oxide), commercially available from TEGO CHEMIE SERVICE GmbH. (7) See Table 1. Imaging and Processing of the Printing Plate Precursor PPP-01 with DEV-01 to DEV-03

The printing plate precursor PPP-01 was exposed with a Creo Trendsetter 3244 (plate-setter, trademark from Creo, Burnaby, Canada), operating at 150 rpm and varying energy densities up to 200 mJ/cm².

After exposure, the plate was processed with the developing solutions DEV-01 to DEV-03 as defined in Table 4.

TABLE 4 Composition of Developing Solutions DEV-01 to DEV-03 DEV-01 DEV-02 DEV-03 INGREDIENTS (g) (g) (g) Na-metasilicate (1) 88 88 88 Crafol AP261 (2) 8.83 8.83 8.83 Synperonic T304 (3) 4.4 4.4 4.4 Block-01 0.175 Block-03 0.31 Water until 1000 1000 1000 (1) Na-metasilicate is sodium metasilicate pentahydrate, commercially available from SILMACO NV (2) Crafol AP261 is alkylether sodium salt, commercially available from COGNIS (3) Synperonic T304 is a block-co-polymer of polyethylene oxide (=PEO) and polypropylene oxide (=PPO) attached to ethylenediamine (=EDA) in a ratio EDA/PEO/PPO of 1/15/14 and having a mean molecular weight of 1600, commercially available from UNIQEMA.

In these experiments, the precursor was exposed at the Right Exposure, hereinafter also referred to as RE. The RE is defined as the exposure energy at which a dot coverage on the plate for a 1×1 checkerboard and for a 8×8 checkerboard matches 50%. The dot coverage was measured with a GretagMacbeth D19C densitometer, commercially available from Gretag-Macbeth AG, with the uncoated support as reference.

The development latitude was determined by developing a 1×1 checkerboard at different developing times. The Right Developing Time, hereinafter also referred to as RDT, is defined whereby the optical density, hereinafter also referred to as OD, on the plate match a 50% dot coverage, measured with a densitometer as described above. (A 50% dot coverage corresponds with an OD of approximately 0.3; the experimentally measured values are indicated in Table 5). The development latitude was measured by the difference between the optical density of a 1×1 checkerboard, developed at a shorter (e.g., −3 seconds) or longer (e.g., +2, +5 or +10 seconds) developing time, and the optical density of a 1×1 checkerboard, developed at the RDT. The lower the absolute value of this difference in OD, the more efficient the additive is working as a blocking agent to reduce the dissolution kinetic of this coating and the more the development latitude is improved.

The results are summarized in Table 5.

TABLE 5 Results of Developing Latitude Comparative Inventive Inventive Example 1 Example 1 Example 2 Precursor PPP-01 PPP-01 PPP-01 Developing solution DEV-01 DEV-02 DEV-03 Right developing time for 1x1 22 s 22 s 27 s screen pattern (RDT) (seconds) OD_((1x1)) at RDT 0.30 0.32 0.29 [(OD_((1x1)) at RDT + 2 s) − −0.11 −0.05 (OD_((1x1)) at RDT)] [(OD_((1x1)) at RDT + 5 s) − −0.14 −0.06 −0.07 (OD_((1x1)) at RDT)] [(OD_((1x1)) at RDT + 10 s) − −0.24 −0.10 −0.13 (OD_((1x1)) at RDT)] [(OD_((1x1)) at RDT − 3 s) − +0.09 +0.03 +0.08 (OD_((1x1)) at RDT)]

The results in Table 5 demonstrate that the improved development latitude for the Inventive Examples 1 and 2 including a blocking agent according to a preferred embodiment of the present invention in the developing solution in comparison with the Comparative Example 1.

Imaging and Processing of the Printing Plate Precursor PPP-01 with DEV-04 to DEV-07

The printing plate precursor PPP-01 was exposed with a Creo Trendsetter 3244 (plate-setter, trademark from Creo, Burnaby, Canada), operating at 150 rpm and an energy density of 100, 110 and 120 mJ/cm².

After exposure, the plate was processed with the developing solutions at a temperature of 25° C. for 15 seconds for DEV-04 and for 16 seconds for DEV-05 to DEV-07. The composition of the developing solutions are defined in Table 6.

TABLE 6 Composition of Developing Solutions DEV-04 to DEV-07 DEV-04 DEV-05 DEV-06 DEV-07 INGREDIENTS (g) (g) (g) (g) Na-metasilicate (1) 177 177 177 177 Crafol AP261 (2) 22.3 22.3 22.3 22.3 Compound-01 0.89 Block-01 0.356 Block-05 0.89 Water 1780 1780 1780 1780 (1) and (2) see Table 4.

The dot coverages of a 8×8 checkerboard, exposed at these three exposure energies, were measured with a GretagMacbeth D19C densitometer with the uncoated support as reference, as described above.

The exposure latitude is determined by the difference between the OD of the 8×8 checkerboard at each exposure energy. The lower this difference in OD, the more efficient of the additive is working as a blocking agent to reduce the dissolution kinetic of this coating and the more the exposure latitude is improved.

The results are summarized in Table 7.

TABLE 7 Results of Exposure Latitude Exposure Developing Developing Energy time OD Precursor solution (mJ/cm²) (seconds) (8x8) Comparative PPP-01 DEV-04 100 15 s 0.180 Example 2 110 15 s 0.137 120 15 s 0.095 Comparative PPP-01 DEV-05 100 16 s 0.204 Example 3 110 16 s 0.089 120 16 s 0.012 Inventive PPP-01 DEV-06 100 16 s 0.300 Example 3 110 16 s 0.301 120 16 s 0.293 Inventive PPP-01 DEV-07 100 16 s 0.340 Example 4 110 16 s 0.330 120 16 s 0.313

The results in Table 7 demonstrate that the improved exposure latitude for the Inventive Examples 3 and 4 including a blocking agent according to a preferred embodiment of the present invention in the developing solution DEV-06 and DEV-07.

The developing solutions DEV-04 and DEV-05 in the Comparative Examples 2 and 3 include no blocking agent and a blocking agent of the prior art respectively, whereby these developing solutions behave a too strong developing property resulting in an OD<0.3 for a 8×8 checkerboard. This drop in OD is much higher with increasing exposure energy. This means that the exposure latitude for these developing solutions is very poor in comparison with the developing solutions of the preferred embodiments of the present invention whereby the change in OD at different exposure energies is very small, resulting in a high exposure latitude.

Preparation of the Printing Plate Precursor PPP-02

The PPP-02 was produced by applying a coating layer defined in Table 8 onto the above described lithographic support. The solvent used to apply the coating is a mixture of 17% by volume of Dowanol PM and 83% by volume of tetrahydrofuran. The coating was applied at a wet coating thickness of 20 μm and then dried at 135° C.

TABLE 8 Composition of the Coating Layer Amount of product INGREDIENTS (g/m²) Binder-01 (25% by weight) (1) 1.00 Crystal Violet (1% by weight) (2) 0.019 Tegoglide 410 (1% by weight) (3) 0.0025 Total dry coating weight (g/m²) 1.02 (1) to (3) see previous tables. Processing of the Printing Plate Precursor PPP-02 in DEV-Reference and DEV-08 to DEV-22

The printing plate precursor PPP-02 includes the coating layer as defined in Table 8 and this layer has an analogous composition as the first coating layer of PPP-01. This layer is soluble in an alkaline solution and is used as a model for testing the efficiency of the blocking agents to reduce the dissolution kinetic of non-exposed areas. Therefore, the precursor PPP-02 was processed, without previously been exposed to IR-laser or heat, in the developing solutions as mentioned in Table 9 and 10 with a dwell time of 20 seconds at a temperature of 25° C. DEV-Reference is a developing solution without addition of a blocking agent or another compound; DEV-08 to DEV-18 are developing solutions including a blocking agent of preferred embodiments of the present invention; and DEV-19 to DEV-22 are developing solutions including another additive compound as defined in Table 9 and 10.

TABLE 9 Composition of the Developing Solutions DEV-Reference and DEV-08 to DEV-22 DEV- DEV-08 to DEV-18 to Reference DEV-17 DEV-22 INGREDIENTS (g) (g) (g) Na-glucoheptonate (1) 5 5 5 Na-metasilicate (2) 102 102 102 Na-silicate solution (3) 10 10 10 Variquat cc 9NS (4) 0.044 0.044 0.044 Triton H-66 (5) 5.8 5.8 5.8 Synperonic T304 (6) 0.141 0.141 0.141 a blocking agent added as 0.01 mol defined in Table 10 a compound added as defined 0.01 mol in Table 10 Water until 1000 1000 1000 (1) Na-glucoheptonate is glucoheptonate sodium salt (2) See previous tables (3) Variquat cc 9NS is a cationic surfactant, commercially available from GOLDSCHMIDT (4) Triton H-66 is an anionic surfactant, commercially available from SEPULCHRE (5) See previous tables.

After processing, the OD is measured with a GretagMacbeth D19C densitometer with the uncoated support as reference, as described above. The difference between the OD of the plate PPP-02, developed with each of the developing solutions DEV-08 to DEV-22, and the OD of the plate PPP-02, developed with DEV-Reference, were calculated. The higher the value of this difference in OD, the more efficient the additive is working as a blocking agent to reduce the dissolution kinetic of this coating.

The blocking efficiency of the different compounds was rated by the difference in OD:

-   Code 0: difference in OD<0.15, means a very poor blocking effect, -   Code 1: difference in OD>0.15 and <0.60, means a moderate blocking     effect, -   Code 2: difference in OD≧0.60 and <1.00, means a good blocking     effect, -   Code 3: difference in OD>1.00, means a high blocking effect.

The results are summarized in Table 10.

TABLE 10 Results of Blocking Efficiency Additive in Code for Developing developing blocking Precursor solution solution efficiency Inventive PPP-02 DEV-08 Block-01 2 Example 5 Inventive PPP-02 DEV-09 Block-02 2 Example 6 Inventive PPP-02 DEV-10 Block-03 3 Example 7 Inventive PPP-02 DEV-11 Block-04 3 Example 8 Inventive PPP-02 DEV-12 Block-05 2 Example 9 Inventive PPP-02 DEV-13 Block-06 1 Example 10 Inventive PPP-02 DEV-14 Block-07 1 Example 11 Inventive PPP-02 DEV-15 Block-08 3 Example 12 Inventive PPP-02 DEV-16 Block-09 3 Example 13 Inventive PPP-02 DEV-17 Block-10 2 Example 14 Comparative PPP-02 DEV-18 Compound-02 0 Example 5 Comparative PPP-02 DEV-19 Compound-03 0 Example 6 Comparative PPP-02 DEV-20 Compound-04 0 Example 7 Comparative PPP-02 DEV-21 Compound-05 0 Example 8 Comparative PPP-02 DEV-22 Compound-06 0 Example 9

The results in Table 10 demonstrate that, in the Inventive Examples 5 to 14, the developers DEV-08 to DEV-17 including a blocking agent of a preferred embodiment of the present invention exhibit an improved efficiency in reducing the dissolution kinetic of this coating. In the Comparative Examples to 9, the developers DEV-18 to DEV-22, including a compound of the prior art, exhibit a poor blocking effect.

Preparation of the Printing Plate Precursors PPP-03 to PPP-07

The PPP-03 to PPP-07 were produced by first applying a first coating layer defined in Table 11 onto the above described lithographic support. The coating was applied at a wet coating thickness of 20 μm and then dried at 135° C. The dry coating weight amounts to 1.02 g/m².

TABLE 11 Composition of the Solution of the First Coating Layer Amount Amount Amount of Amount of of Amount of of product product product product product for for for for for PPP-03 PPP-04 PPP-05 PPP-06 PPP-07 INGREDIENTS (g) (g) (g) (g) (g) Dowanol PM (1) 160.74 160.45 160.45 160.45 160.45 THF (2) 536.04 535.96 535.96 535.96 535.96 Binder-01 211.37 211.62 211.62 211.62 211.62 (25 wt. %) (3) Block-04 1.35 Block-11 1.35 Block-03 1.35 Block-01 1.35 Crystal 81.05 81.14 81.14 81.14 81.14 Violet (1 wt. %) (4) Tegoglide 410 10.81 10.82 10.82 10.82 10.82 (1 wt. %) (5) (1) to (5) see previous tables.

On the first coated layer, a second layer as defined in Table 12 was coated at a wet coating thickness of 25 μm and dried at 135° C. The dry coating weight amounts to 0.80 g/m².

TABLE 12 Composition of the Solution of Second Coating Layer Amount of product for PPP-03 to PPP-07 INGREDIENTS (g) Dowanol PM (1) 235.0 Alnovol SPN452 (40% by weight) (2) 117.11 TMCA (10% by weight) (3) 60.34 SOO94 (4) 2.41 Crystal Violet (1% by weight) (5) 70.99 Tegowet 265 (1% by weight) (6) 9.94 Tegoglide 410 (1% by weight) (7) 32.65 Butanone 471.55 (1) to (5) see previous tables. Imaging and Processing of Printing Plate Precursor PPP-03 to PPP-07

The printing plate precursors PPP-03 to PPP-07 were, in a first experiment, exposed with a Creo Trendsetter 3244 (plate-setter, trademark from Creo, Burnaby, Canada), operating at 150 rpm at the Right Exposure (RE) as defined above and developed with DEV-01 as defined above in Table 4.

In a second experiment, the printing plate precursors PPP-03 to PPP-07 were exposed by a 1×1 checkerboard at an exposure energy which is 20% lower than the RE value as determined above and, also, at an exposure energy which is 20% higher than the RE value. After developing, the dot coverage for the under-exposure by 20% and for the over-exposure by 20% was measured by the OD as defined above. The exposure latitude was determined by the difference between the dot coverage by the 20% under-exposure and the dot coverage by the 20% over-exposure. The lower the absolute value of this difference, the more efficiently the dissolution kinetic of the coating is reduced and the more the exposure latitude is improved.

The results are summarized in Table 13.

TABLE 13 Results of Exposure Latitude RE-value [Dot coverage_(at RE−20%)] − EXAMPLES PPP-type (mJ/cm²) [Dot coverage_(at RE+20%)] Comparative PPP-03 89 19 Example 10 Inventive PPP-04 124 13 Example 15 Inventive PPP-05 132 12 Example 16 Inventive PPP-06 110 8 Example 17 Inventive PPP-07 121 15 Example 18

The results in Table 13 demonstrate an improved exposure latitude for the Inventive Examples 15 to 18, including a blocking agent according to a preferred embodiment of the present invention, whereby the difference between the dot coverage by the 20% under-exposure and the dot coverage by the 20% over-exposure is lower than in the Comparative Example 10.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. A method for making a lithographic printing plate comprising the steps of: providing a heat-sensitive lithographic printing plate precursor including a support having a hydrophilic surface or which is provided with a hydrophilic layer, and a heat-sensitive coating including an infrared absorbing agent and an alkaline soluble oleophilic resin on the support; image-wise exposing the precursor with IR-radiation or heat; and developing the image-wise exposed precursor with an alkaline developing solution including a compound having at least two onium groups such that the heat-sensitive coating is dissolved in the alkaline developing solution at a higher dissolution rate in areas which are exposed to the IR-radiation or heat than in areas which are not exposed to the IR-radiation or heat; wherein the compound having at least two onium groups has a structure according to Formula I, II, or III:

Z^(a)-L-Z^(b),  Formula III; Q is a divalent group when e and f are 0, a trivalent group when the sum of e and f is 1, or a tetravalent group when e and f are both 1; Z^(a) to Z^(d) independently are onium groups; L and L^(a) to L^(d) independently are linking groups selected from the group consisting of an arylene group, an arylene-alkylene group, an oxy-alkylene group, an oxy-arylene group, an oxy-heteroarylene group, and a combination of two or more of these groups; Z^(x)-L^(x) represents a repeating unit wherein Z^(x) and L^(x) represent respectively an onium group and a linking group for each x-value; a, b, c, d, e, and f independently are 0 or 1; x is an integer ranging between 1 and 10; and the precursor includes a compound having a structure according to Formula I, II, or III, and during the developing step, at least a portion of the compound is removed from the precursor.
 2. A method according to claim 1, wherein the onium group is selected from the group consisting of primary amine salts, secondary amine salts, tertiary amine salts, quaternary ammonium salts, phosphonium salts, and sulphonium salts.
 3. A method according to claim 1, wherein during or after the developing step, a replenishing solution including a compound having a structure according to Formula I, II, or III is added to the developing solution.
 4. A method according to claim 1, wherein the alkaline developing solution including the compound having a structure according to Formula I, II, or III further includes a silicate or metasilicate. 